The present invention relates to a dynamic range compression circuit appropriate for dynamic range compression on audio signals and a class D amplifier including the dynamic range compression circuit.
Audio apparatuses often use dynamic range compression technology for enabling reproduced sounds of input audio signals having a wide dynamic range from a speaker to be easily heard by a listener. The dynamic range compression is a technology which outputs an input audio signal having a wide dynamic range as a reproduced sound having the volume of sound in a range comfortably audible for listeners by increasing a gain of amplification in a range in which the volume of sound of an input audio signal is low, and decreasing a high gain of amplification in a range in which the volume of sound of the input audio signal is high when the input audio signal is amplified and reproduced (see, for example, JP-A-5-110362, JP-A-2005-302186, and JP-A-2007-104407).
In the related art, a voltage lower than an output limit voltage VOLMT by a predetermined voltage, which is a limit for preventing a clip from being generated in an output signal waveform of an amplifier, is set as a threshold, and in range in which the amplitude of an output signal VO of the amplifier is the threshold or more, the gain of the amplifier is reduced so as to perform dynamic range compression. Therefore, in a case where an input-stage maximum voltage VIMAX, which is the maximum amplitude of an input signal transmitted from a circuit at the previous stage to the amplifier, is large, as shown in FIG. 12A, before the amplitude of the input signal VI of the amplifier reaches the input-stage maximum voltage VIMAX, the amplitude of the output signal VO of the amplifier reaches the output limit voltage VOLMT, such that a further increase in amplitude of the input signal VI cannot be reflected as an increase in amplitude of the output signal VO. Further, in a case where the input-stage maximum voltage VIMAX is low, as shown in FIG. 12B, even if the input signal VI of the amplifier reaches the input-stage maximum voltage VIMAX, the amplitude of the output signal VO of the amplifier becomes lower than the output limit voltage VOLMT.